1. Field of the Invention
The invention is related to hypodermic syringes. More specifically, the invention is related to devices for controlling or slowing the rate at which a hypodermic syringe dispenses its contents when manually operated.
2. Description of the Related Art
Most syringes are made of plastic. Plastic syringes are mass-produced and disposable. A typical syringe includes a plastic barrel and a plastic plunger having an elastomeric seal, or stopper, at its distal end inside the barrel. The plunger is reciprocatable inside the barrel. The barrel typically has a pair of flanges at its proximal end under which the user places his index and middle fingers. The plunger typically has a head, or thumbpad, at its proximal end upon which the user places his thumb.
To dispense medicine contained in the syringe barrel, the user increases the pressure applied by his or her thumb to the plunger head, while applying opposing (supporting) pressure with two fingers on the flanges of the barrel. Owing to the pressure of the thumb on the head of the plunger, the plunger advances into the barrel and forces the medication out via the needle. Syringe lubricant is included by the syringe manufacturer to allow the stopper to slide more easily within the barrel. Polydimethyl siloxane fluid is a typical syringe lubricant.
The benefits of disposable plastic syringes include low cost and increased patient safety. However, along with the benefits of plastic syringes has come an inherent problem: It is commonly and aptly described as “stick-slip” behavior. The “stick-slip” effect makes it quite difficult to inject medication slowly.
What are the benefits of slow injection? The first is comfort. Regardless of medication, a slow injection is typically less painful than a rapid injection. For some medications (e.g. heparin) sudden injection can actually be quite uncomfortable for the patient. The second benefit of slow injection is, in some cases, better end results, probably owing to better assimilation of slowly injected medication by cells in the injected tissue. Increased effectiveness associated with slow injection rates has been demonstrated, for examples, in certain anaesthetic injections and in some inoculation procedures. In such cases the advantage to the individual patient of a slowly delivered, finely incremented injection include either 1) a reduced dose to produce the same benefit or 2) a quicker or more vigorous benefit produced by the same dose.
Slow injection offers economies of scale. In applications where it can be shown to produce an enhanced end result a reduced dosage might be prescribed. For an individual patient using medicine to treat a chronic condition, injecting daily for example, a slight daily reduction in dosage could add up to a significant reduction in the total amount of medicine injected over a period of years. Similarly, in a mass inoculation program conducted with limited supplies of a vaccine, there could be a social gain based simply on the conservation of vaccine. At present, the possibilities of such economies of scale have not been fully explored since costly and specialized motor-driven syringes must be used for slow metering of injected volumes.
Why do plastic syringes make it difficult to perform slow injections? First they stick and then, suddenly, they slip. To start an injection, significant thumb pressure, called the “break out” force, must be applied to the syringe plunger to overcome static friction and put the plunger in motion. However, in the instant the threshold of the “break out” force is exceeded the plunger friction decreases dramatically and without warning. As a result the syringe plunger, which is still receiving very strong pressure from the thumb, suddenly surges into the syringe barrel.
The plastic syringe plunger's transition between “stick” and “slip” is so very quick that the human being operating the syringe is usually incapable of backing off the thumb pressure in time to prevent the sudden downward surge of the plunger. The result is that fluid is dispensed from the syringe and into the tissue as a large bolus, or slug, of medication.
A slow, steadily progressing injection stroke is difficult for anyone to achieve with a plastic syringe, and it is especially difficult for non-professionals who may be required to self-inject. Stick-slip behavior is a particularly noticeable problem if only a few units are required to be manually injected slowly. For example, for a 5-unit manual injection from a 50-unit capacity U-100 type Becton Dickinson disposable insulin syringe, the entire injection of 5 units may be delivered in just two abrupt surges, owing to the stick-slip properties of the syringe. Thus, the syringe delivers to the tissues two successive boluses of medication, one right after the other—rather than a slow, incrementally metered stream.
Stick-slip behavior arises from the interaction of the elastomer used to manufacture the syringe seal, or stopper; the syringe lubricant; and the syringe cylinder. To some degree, it is characteristic of all disposable plastic syringes. Stick-slip is a velocity dependent phenomenon, and it is most troublesome in slow injections. In addition to interfering with slow dispensing of medicine from a plastic syringe, the inherent stick-slip action of a plastic syringe makes it tricky to precisely and quantitatively load the syringe, particularly if fractional volume units are desired. The piston has a tendency to “jump” past the desired increment mark or position.
Another problem arises from the wrong kind of leverage. In the example of the poorly controlled 5-unit injection noted above, the thumb, pivoting at a center located at its base joint at the wrist, quickly traverses through a tiny angular displacement of just 2-3 degrees in delivering a dose of 5 units. The thumb, like most anatomical levers, is a third class lever. It operates at a mechanical disadvantage. Muscular effort is sacrificed in a lever of this type in order to gain distance and, therefore, speed. The thumb is configured for sudden movement. A tiny angular displacement about the center (that is, the joint) located at the base of the thumb results in a large, sudden displacement of the syringe plunger by the thumbtip. The longer the thumb, the faster the thumbtip will move for a given angular displacement. From the standpoint of fine control and slow injection, this geometry is certainly not helpful. Moreover, the high breakout force required to overcome the molecular interaction between the elastomeric stopper and the plastic syringe barrel begs for an increase in mechanical advantage—not speed.
Prior efforts to ameliorate the problem include chemical modification of the crosslinking of dimethyl siloxane syringe lubricants. The idea was to diminish static friction, that is, to reduce the “stick” component of the stick-slip effect. Changing the lubricant chemistry reportedly helped, but different medications may require different lubricants for optimum results. Changes in lubricant chemistry may also be required to optimize injections at different rates. In any event, a better lubricant is only a small initial step toward a solution. Lubrication can only alleviate, to some degree, the “stick”, or static friction problem. But in a manual syringe the “slip” and the surge injection it produces must also be addressed.
A low cost, widely applicable solution that is independent of injection rate, and of the specific type of medication to be injected is needed. The inventive solution needs to do two things: 1) overcome the syringe's “stick”, or static friction, and then 2) limit or actively arrest the subsequent “slip” and surge.